NIL-REFLEXIVE RINGS HANDAN KOSE, BURCU UNGOR, AND ABDULLAH HARMANCI

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1 Commun. Fac. Sci. Univ. Ank. Sér. A1 M ath. Stat. Volum e 65, N um b er 1, Pages D O I: /C om m ua1_ ISSN NIL-REFLEXIVE RINGS HANDAN KOSE, BURCU UNGOR, AND ABDULLAH HARMANCI A. In this paper, we deal with a new approach to reflexive property for rings by using nilpotent elements, in this direction we introduce nil-reflexive rings. It is shown that the notion of nil-reflexivity is a generalization of that of nil-semicommutativity. Examples are given to show that nil-reflexive rings need not be reflexive and vice versa, and nil-reflexive rings but not semicommutative are presented. We also proved that every ring with identity is weakly reflexive defined by Zhao, Zhu and Gu. Moreover, we investigate basic properties of nil-reflexive rings and provide some source of examples for this class of rings. We consider some extensions of nil-reflexive rings, such as trivial extensions, polynomial extensions and Nagata extensions. 1. I Throughout this paper all rings are associative with identity unless otherwise stated. Mason introduced the reflexive property for ideals, and this concept was generalized by some authors, defining idempotent reflexive right ideals and rings, completely reflexive rings, weakly reflexive rings see namely, [6], [9], [13]. Let R be a ring and I be a right ideal of R. In [13], I is called a reflexive right ideal if for any x, y R, xry I implies yrx I. The reflexive right ideal concept is also specialized to the zero ideal of a ring, namely, a ring R is called reflexive [13] if its zero ideal is reflexive. Reflexive rings are generalized to weakly reflexive rings in [13]. The ring R is said to be weakly reflexive if arb = 0 implies bra is nilpotent for a, b R and all r R. Motivated by the works on reflexivity, in this note we study the reflexivity property in terms of nilpotent elements, namely, nil-reflexive rings. It is shown by examples that the class of reflexive rings and the class of nil-reflexive rings are incomparable. In [13], a ring R is called completely reflexive if for any a, b R, ab = 0 implies ba = 0. Completely reflexive rings are called reversible by Cohn in [4] and also studied in [5]. The rings without nonzero nilpotent Received by the editors: Feb. 26, 2015, Accepted: Dec. 30, Mathematics Subject Classification. 13C99, 16D80, 16U80. Key words and phrases. Reflexive ring, completely reflexive ring, weakly reflexive ring, nilreflexive ring, semicommutative ring, nil-semicommutative ring. c 2016 A nkara U niversity Communications de la Faculté des Sciences de l Université d Ankara. Séries A1. M athematics and Statistics. 19

2 20 HANDAN KOSE, BURCU UNGOR, AND ABDULLAH HARMANCI elements are said to be reduced rings. Reduced rings are completely reflexive and every completely reflexive ring is semicommutative, i.e. according to [12], a ring R is called semicommutative if for all a, b R, ab = 0 implies arb = 0. This is equivalent to the definition that any left right annihilator of R is an ideal of R. In [3], semicommutativity of rings is generalized to nil-semicommutativity of rings. A ring R is called nil-semicommutative if a, b R satisfy that ab is nilpotent, then arb nilr for any r R where nilr denotes the set of all nilpotent elements of R. Clearly, every semicommutative ring is nil-semicommutative. In this paper it is proved that the class of nil-reflexive rings lies strictly between the classes of nil-semicommutative rings and weakly reflexive rings. We first summarize the contents of the sections of this paper. First section is the introduction. In the second section, we investigate the structure of nil-reflexive rings, and some basic characterizations of these rings are obtained. We also deal with relations between nil-reflexive rings and certain classes of rings. We present some examples to illustrate nil-reflexive rings. Examples are given to show that the notions of reflexive rings and nil-reflexive rings do not imply each other. Nilreflexive rings share a number of important properties with other classes of rings. For instance, among other interesting results, we prove every semicommutative ring is nil-reflexive. For a nil ideal I of a ring R, it is proved that R is nil-reflexive if and only if R/I is nil-reflexive. Also if R is a nil-reflexive ring, then T n R, S n R and V n R see below for the definitions are nil-reflexive. It is shown that every corner ring of any nil-reflexive ring inherits the nil-reflexive property. On the other hand, we determine abelian semiperfect nil-reflexive rings, they are exactly in the form of a finite direct sum of local nil-reflexive rings. In the third section, we study some extensions of nil-reflexive rings and it is proved that a ring R with a multiplicatively closed subset U consisting of some central elements is nil reflexive if and only if U 1 R is nil-reflexive; for a ring R, R[x] is nil-reflexive if and only if R[x; x 1 ] is nil-reflexive; if R is a nil-reflexive and Armendariz ring, then R[x] is a nil-reflexive ring. Also, R is nil-reflexive if and only if its trivial extension T R, R is nil-reflexive. We also deal with the Nagata extension N[R, R; α] of a commutative ring R in terms of nil-reflexivity. In what follows, N, Z and Q denote the set of natural numbers, the ring of integers and the ring of rational numbers, and for a positive integer n, Z n is the ring of integers modulo n. For a positive integer n, let Mat n R denote the ring of all n n matrices and T n R the ring of all n n upper triangular matrices with entries in R. We write R[x], UR, P R, and S n R V n R for the polynomial ring over a ring R, the set of invertible elements, the prime radical of R, and the subring consisting of all upper triangular matrices over a ring R with equal main diagonal every diagonal entries, respectively.

3 NIL-REFLEXIVE RINGS N-R R In this section, we introduce nil-reflexive rings, and investigate basic properties of this class of rings. We also study the relations between nil-reflexive rings and some certain classes of rings. Definition 2.1. A ring R is said to be nil-reflexive if for any a, b R, arb being nilpotent implies that bra is nilpotent for all r R. In the next, we provide some examples for nil-reflexive rings. The third example in the following also shows that nil-reflexive rings need not be reflexive. In [6, Theorem 2.6], Kwak and Lee proved that R is a reflexive ring if and only if Mat n R is a reflexive ring for all n 1. However, this is not the case in nil-reflexivity of R. There are nil-reflexive rings over which matrix rings need not be nil-reflexive as shown below. Examples Let R be a ring with nilr an ideal of R. Then R is nil-reflexive. 2 For any reduced ring S, the ring T n S is nil-reflexive. However, the ring of all 2 2 matrices over any field is not nil-reflexive. 3 Let R be a reduced ring. Consider the ring a a 12 a 13 a 1n 0 a a 23 a 2n S n R =. a, a ij R; 1 i, j n. 0 a Then S n R is not reflexive when n 4, but S n R and R are nil-reflexive for all n 1. Proof. 1 Let a, b R. Assume that arb is nilpotent for all r R. Then ab nilr and so brabra nilr. Hence bra is nilpotent for all r R. Hence R is nil-reflexive. nilr R R R 0 nilr R R 2 For a ring R, by [2], nilt n R = Let 0 nilr S be a reduced ring. Then nils = 0 and so nilt n S is an ideal. By 1, 0 1 T n S is nil-reflexive. Let A =, B = Mat 2 F where F is 0 1

4 22 HANDAN KOSE, BURCU UNGOR, AND ABDULLAH HARMANCI a b 0 d a field. For any C = Mat 2 F, ACB = is nilpotent for c d all C Mat 2 F, but for C = Mat 2 F, BCA = is not nilpotent. Therefore Mat 2 F is not nil-reflexive. 3 It is proved in [6] that S n R is not reflexive when n 4. Since R is reduced, R is nil-reflexive. Note that a a 12 a 13 a 1n 0 a a 23 a 2n nils n R =. a nilr, a ij R, 1 i, j n. 0 a The ring R being reduced implies that nils n R is an ideal. By 1, S n R is nil-reflexive. Lemma 2.3. For a ring R, consider the following conditions. 1 R is nil-reflexive. 2 If ARB is a nil set, then so is BRA for any subsets A, B of R. 3 If IJ is nil, then JI is nil for all right or left ideals I, J of R. Then Proof. 1 2 Assume that R is a nil-reflexive ring and ARB is a nil set. For any a A, b B, arb is nilpotent for all r R, then bra is nilpotent. This implies that BRA is nil. 2 3 Let I and J be any right ideals of R such that IJ is nil. Since IR I, IRJ is nil. By 2, JRI is nil. Since JI JRI, we get JI is nil. Assume that I and J be any left ideals of R such that IJ is nil. Since RJ J and then IRJ IJ, IRJ is nil. By 2, JRI is nil. Since JI JRI, we get JI is nil. Lemma 2.4. The following conditions are equivalent for a ring R. 1 ar nilr for any a nilr. 2 Ra nilr for any a nilr. Proof. 1 2 Assume that ar nilr for all r R, for any a nilr. Let ar n = 0 for some positive integer n. Then ra n+1 = 0, hence ra is nilpotent. Thus Ra nilr. Similarly, we can show 2 1.

5 NIL-REFLEXIVE RINGS 23 The next result gives a source of nil-reflexive rings. Proposition 2.5. Let R be a ring such that ar nilr for any a nilr. Then R is nil-reflexive. Proof. Assume that for a, b R, arb nilr for any r R. So ab nilr. By hypothesis, abr nilr. Then there exists m N such that abr m = 0. Hence br abrabrabr... abr }{{} a = bram+1 = 0. So bra nilr for any r R. [13, Example 2.1] shows that any semicommutative ring need not be reflexive, but this is not the case when we deal with nil-reflexive rings. It can be observed that every semicommutative ring is nil-reflexive as a consequence of Proposition 2.5. But we give its direct proof in the next. Lemma 2.6. If R is a semicommutative ring, then it is nil-reflexive. Proof. Assume that R is semicommutative and arb is nilpotent for a, b R and for all r R. Let r R with arb n = 0 for some positive integer n. arbarbarb... arb = }{{} n-times In 2.1 insert b before r, and a after r to have Replacing ba in 2.2 by bra to obtain abrababrababrab... babrab = abrabrabrabrabrabra... brabrab = Multiplying the equation 2.3 by br from the left and by ra from the right, we get brabrabrabrabrabrabr... brabrabra = bra 2n+1 = Hence R is nil-reflexive. In [13], weakly reflexive rings are studied in detail for rings having an identity. However weakly reflexive rings are nothing but all rings with identity as it is shown below. Lemma 2.7. Every ring with identity is weakly reflexive. Proof. Let a, b R with arb = 0 for all r R. Then ab = 0 and so bra 2 = brabra = 0 for all r R. Hence bra is nilpotent for all r R. Thus R is weakly reflexive.

6 24 HANDAN KOSE, BURCU UNGOR, AND ABDULLAH HARMANCI Lemma 2.6 and the next result show that the class of nil-reflexive rings lies between the classes of semicommutative rings and weakly reflexive rings. It is known that every completely reflexive ring is semicommutative and so nil-reflexive by Lemma 2.6. In the following, we give the direct proof of this fact for the sake of completeness. Lemma 2.8. If R is a completely reflexive ring, then it is nil-reflexive. Proof. Let R be a completely reflexive ring and a, b R. Assume that arb is nilpotent for all r R. Then there exists n N such that arb n = 0. For any r R, we apply successively completely reflexivity of R to get 0 = ababab... ababr = brababab... aba = abrababab... ababr = brabrababab... aba = brabrababab... aba = abrabrabab... ab = abrabrabab... abr = brabrabrabab... a = = bra n. Therefore R is nil-reflexive. Now we shall give an example to show that there exists a nil-reflexive ring which is not reflexive. Also reflexive rings may not be nil-reflexive either as shown below. Example 2.9. There exists a nil-reflexive ring which is neither reflexive nor semicommutative. Proof. Let R be a reduced { ring. By Examples } 2.22, T 2 R is nil-reflexive. On the 0 b other hand, nilt 2 R = b R. Consider, 0 1 R R T 2 R. Then = 0, 0 R for R. This shows that T 2 R is not reflexive. T 2 R is also not semicommutative. For if, A =, B = and C = T 2 R, then AB = 0 but ACB 0. Example Consider the ring Mat 2 F where F is a field. Since F is a semiprime ring, Mat 2 F is also semiprime due to [7, Proposition 10.20]. This implies that Mat 2 F is a reflexive ring, it is also weakly reflexive. On the other hand, Mat 2 F is not nil-reflexive by Examples Note that there are nil-reflexive rings but not completely reflexive as the following example shows.

7 NIL-REFLEXIVE RINGS 25 Example Let F be a field. Then T 2 F is nil-reflexive which is not completely reflexive. We now observe some relations among nil-reflexive rings, nil-semicommutative rings and semiprime rings. According to next result, the class of nil-reflexive rings is weaker than that of nil-semicommutative rings. Proposition Every nil-semicommutative ring is nil-reflexive. Proof. Let R be a nil-semicommutative ring. Let a, b R with arb nilr for all r R. In particular, for r = 1, we have ab nilr. So ba nilr. Since R is nil-semicommutative, bra nilr for any r R. Thus R is a nil-reflexive ring. It is easy to check that every semiprime ring is reflexive see, for example [6]. We have given an example showing that this is not the case for nil-reflexive rings in Example There are also nil-reflexive rings but not semiprime. Example Let D be a division ring. Consider the ring a b c R = 0 a d a, b, c, d D. a Then R is nil-reflexive but not semiprime. Proof. R is not semiprime since the set consisting of all main diagonal off elements of R is a nonzero nilpotent ideal. On the other hand, R is nil-reflexive by Examples In the next, we investigate the relations between a ring R and R/I for some ideal I of R in terms of nil-reflexivity. Lambek called a ring R symmetric [8] provided that abc = 0 implies acb = 0 for all a, b, c R. By Lemma 2.6, every symmetric ring is nil-reflexive. Proposition Let R be a ring. Then the following hold. 1 Let I be an ideal of R contained in nilr. Then R is nil-reflexive if and only if R/I is nil-reflexive. 2 If R is symmetric, I is an ideal of R and I is a right annihilator I = r R S for some nonempty subset S of R, then R/I is nil-reflexive.

8 26 HANDAN KOSE, BURCU UNGOR, AND ABDULLAH HARMANCI Proof. 1 = " Let ā, b R/I with ā r b nilr/i for all r R/I. Then there exists n N such that ā r b n = 0. So arb n I. Since I nilr, arb n nilr. Hence arb nilr. Since R is nil-reflexive, bra nilr. Thus b rā nilr/i for all r R/I. Therefore R/I is nil-reflexive. =" Let a, b R and suppose that arb nilr for all r R. Then ā r b nilr/i and so b rā nilr/i since R/I is nil-reflexive. There exists m N such that b rā m = 0. This shows that bra m I. Since I nilr, bra m nilr. So there exists n N such that bra m n = 0 and so bra nilr. This implies that R is nil-reflexive. 2 Let a, b R with arb nil R/I for all r R/I. There exists a positive integer t such that arb t I and so Sarb t = 0. Hence 0 = Sarb t = S arbarbarb... arbarbarb }{{} t times = Sarbarbarb... arbarbarbb = Sbarbarbarb... arbarbarb = Sbarbarbarb... arbarbarbra = Sbrabarbarbarb... arbarbar = Sbrabarbarbarb... arbarbar = Sbrabarbarbarb... arbarbra = Sbrabrabarbarbarb... arbar. We continue in this way to have Sbra t+1 = 0. Hence bra t+1 I. This shows that bra is nilpotent for each r R/I. Thus R/I is nil-reflexive. Now we give some characterizations of nil-reflexivity by using the prime radical of a ring, upper triangular matrix rings and polynomial rings. Corollary A ring R is nil-reflexive if and only if R/P R is nil-reflexive. Proof. Since every element of P R is nilpotent, it follows from Proposition Proposition A ring R is nil-reflexive if and only if T n R is nil-reflexive, for any positive integer n. Proof. Let A = a ij, B = b ij T n R, with ACB nilt n R for all C = c ij T n R, where 1 i j n. Then we have a ii c ii b ii nilr for any 1 i n. Since R is nil-reflexive, b ii c ii a ii nilr. So it follows from BCA nilt n R

9 NIL-REFLEXIVE RINGS 27 that T n R is nil-reflexive. Conversely, let a, b R with arb nilr for all r a 0 r 0 b R. Then nilt n R. Since T n R is nil-reflexive, b 0 r 0 a nilt n R. 0 0 So bra nilr. Therefore R is nil-reflexive. Proposition A ring R is nil-reflexive if and only if R[x]/x n is nil-reflexive for any n 1 where x n is the ideal generated by x n in R[x]. Proof. Note that for n = 1, R[x]/x = R and for n 2, a 1 a 2 a 3 a n 1 a n 0 a 1 a 2 a n 2 a n 1. R[x]/x n a.. 1 an 3 a n 2 = a i R, 1 i n = V n R 0 a 1 a a 1 and V n R is a subring of T n R. Therefore a 1 a 2 a 3 a n 1 a n 0 a 1 a 2 a n 2 a n 1. a.. 1 an 3 a n 2 nilv n R = a 1 a a 1 a 1 nilr, a 2,..., a n R. The proof is completed as in the proof of Proposition 2.16.

10 28 HANDAN KOSE, BURCU UNGOR, AND ABDULLAH HARMANCI Theorem Let I and K be ideals of a ring R. Assume that R = I K is a ring direct sum of I and K. Then R is nil-reflexive if and only if I and K are nil-reflexive rings. Proof. Note that R = I K is a ring direct sum of ideals I and K. Then I and K become rings with identity. Suppose that R is nil-reflexive. Let x, y I with xiy nilpotent for all i I. Then x, 0i, ky, 0 is nilpotent for all i, k R. By supposition, y, 0i, kx, 0 is nilpotent for all i, k R. Hence yix is nilpotent for all i I or I is nil-reflexive. A similar discussion proves that K is also nil-reflexive. Conversely, assume that I and K are nil-reflexive rings. Let x, y, x, y R with x, yx, y x, y nilpotent for all x, y R. Then xx x is nilpotent for all x I and yy y is nilpotent for all y K. By assumption, x x x is nilpotent for all x I and y y y is nilpotent for all y K. Then x, y x, y x, y is nilpotent for all x, y R. Hence R is nil-reflexive. Proposition Finite product of nil-reflexive rings is nil-reflexive. Proof. Let {R i } i I be a class of nil-reflexive rings for an indexed set I = {1, 2,..., n} where n N. By [10, Proposition 2.13], nil n R i = n nilr i. Suppose that for any a 1, a 2,..., a n, b 1, b 2,..., b n n R i, for all r 1, r 2,..., r n i=1 i=1 i=1 n a 1, a 2,..., a n r 1, r 2,..., r n b 1, b 2,..., b n nil R i n i=1 i=1 R i. Then we have a i r i b i nilr i for each i = 1, 2,..., n. Since R i is nil-reflexive, b i r i a i nilr i for each 1 i n. Therefore b 1, b 2,..., b n r 1, r 2,..., r n a 1, a 2,..., a n nil n R i. In the next result it is presented that any corner ring of a nil-reflexive ring inherits the nil-reflexivity property. But the nil-reflexivity property is not Morita invariant because of Examples Proposition Let R be a ring and e 2 = e R. If R is nil-reflexive, then so is ere. Proof. Let exe, eye ere with exeereeye nilere for all ere ere. Then there exists m N such that exeereeye m = 0. Hence exereye nilr. i=1

11 NIL-REFLEXIVE RINGS 29 Since R is nil-reflexive, we have eyerexe nilr. nilere. Thus eyeereexe Corollary For a central idempotent e of a ring R, er and 1 er are nil-reflexive if and only if R is nil-reflexive. Proof. Assume that er and 1 er are nil-reflexive. Since the nil-reflexivity property is closed under finite direct products, R = er 1 er is nil-reflexive. The converse is trivial by Proposition By Examples 2.22, for any positive integer n, there are rings R for which Mat n R can not be nil-reflexive. However the converse statement holds as the next result shows. Corollary Let R be a ring. n N, then R is a nil-reflexive ring. If Mat n R is a nil-reflexive ring for some Proof. Let E 11 denote the matrix unit whose 1, 1 entry is 1 and all other entries are zero. Assume that Mat n R is nil-reflexive. Then R = RE 11 = E 11 Mat n RE 11 is nil-reflexive by Proposition Recall that a ring R is said to be abelian if every idempotent is central, that is, ae = ea for any e 2 = e, a R. There exists a nil-reflexive ring which is not abelian as shown below. Example Let F be a field. The ring T 2 F is nil-reflexive by Example 0 y But for an idempotent E = T 2 F and for A = E 12 T 2 F, 0 1 EA AE. Thus T 2 F is not an abelian ring. We close this section by determining abelian semiperfect nil-reflexive rings. Theorem Let R be a ring. Consider the following statements. 1 R is a finite direct sum of local nil-reflexive rings. 2 R is a semiperfect nil-reflexive ring. Then 1 2. If R is abelian, then 2 1. Proof. 1 2 Assume that R is a finite direct sum of local nil-reflexive rings. Then R is semiperfect because local rings are semiperfect and a finite direct sum of semiperfect rings is semiperfect, and moreover R is nil-reflexive by Proposition

12 30 HANDAN KOSE, BURCU UNGOR, AND ABDULLAH HARMANCI Suppose that R is an abelian semiperfect nil-reflexive ring. Since R is semiperfect, R has a finite orthogonal set {e 1, e 2,..., e n } of local idempotents whose sum is 1 by [1, Theorem 27.6], say 1 = e 1 + e e n such that each e i Re i is a local ring where 1 i n. The ring R being abelian implies e i Re i = e i R. Each e i R is a nil-reflexive ring by Proposition Proposition E N-R R Hence R is nil-reflexive by In this section, we consider some extensions of nil-reflexive rings and characterize nil-reflexive rings from various aspects. Let R be a ring and U be a multiplicative closed subset of R consisting of some central regular elements, that is, for any element u U, ur = 0 implies that r = 0 and u is in the center of R. Consider the ring U 1 R = {u 1 r u U, r R}. In the following, we obtain a characterization of nil-reflexivity of the ring U 1 R. Proposition 3.1. A ring R is nil-reflexive if and only if U 1 R is nil-reflexive. Proof. Assume that R is a nil-reflexive ring. Let u 1 a, v 1 b U 1 R be such that u 1 as 1 rv 1 b is nilpotent for all s 1 r U 1 R. Since u 1 as 1 rv 1 b = usv 1 arb and usv is central and invertible, arb is nilpotent for all r R. By assumption bra is nilpotent for all r R. It gives rise to v 1 bs 1 ru 1 a is nilpotent for all s 1 r U 1 R. So U 1 R is nil-reflexive. Conversely, suppose that U 1 R is nil-reflexive. Let a, b R with asb nilpotent for each s R. Then for any u U, u 1 au 1 su 1 b = u 3 asb is nilpotent for each u 1 s U 1 R. By supposition u 1 bu 1 su 1 a = u 3 bsa is nilpotent for each u 1 s U 1 R. Since u is central invertible, bsa is nilpotent for each s R. This completes the proof. Corollary 3.2. For a ring R, R[x] is nil-reflexive if and only if R[x; x 1 ] is nilreflexive. Proof. Let U = {1, x, x 2,... }. Clearly, U is a multiplicatively closed subset of R[x]. Since R[x; x 1 ] = U 1 R[x], the ring R[x; x 1 ] is nil-reflexive by Proposition 3.1. In [12], a ring R is said to be Armendariz if whenever two polynomials fx = n i=0 a ix i, gx = m j=0 b jx j R[x] satisfy fxgx = 0, then a i b j = 0 where

13 NIL-REFLEXIVE RINGS 31 0 i n, 0 j m. There are nil-reflexive rings but not Armendariz as the following example shows. Example 3.3. Let F be a field and consider the ring T 2 F. Then by Example 2.22, T 2 F is nil-reflexive. On the other hand, let fx and gx be given by fx = + x, gx = + x T 2 F [x] Then fxgx = 0, but 0. Therefore T 2 F is not an Armendariz ring. Theorem 3.4. If R is a nil-reflexive Armendariz ring, then R[x] is a nil-reflexive ring. Proof. Let fx = n i=0 a ix i, gx = m j=0 b jx j R[x] such that fxhxgx nilr[x], for all hx = t k=0 c kx k R[x]. Since R is Armendariz, by [2, Corollary 5.2], we have nilr[x] = nilr[x]. We get a i c k b j nilr, 0 i n, 0 j m, 0 k t. Since R is nil-reflexive, b j c k a i nilr, 0 i n, 0 j m, 0 k t. So gxhxfx nilr[x]. Let R be a ring and M an R-bimodule. Recall that the trivial extension of R by M is the ring T R, M = R M with the usual addition and the following multiplication {r 1, m 1 r 2, m 2 = r 1 r 2, r} 1 m 2 + m 1 r 2. This is isomorphic to the r m matrix ring r R, m M with the usual matrix operations. 0 r Theorem 3.5. A ring R is nil-reflexive if and only if T R, R is nil-reflexive. a b x y u v Proof. = Let A =, X =, B = T R, R 0 a 0 x 0 u with AXB nilpotent. Then axu is nilpotent for all x R. By hypothesis, uxa is 0 nilpotent, say uxa t = 0. Then BXA t =. Hence BXA t 2 = 0. Thus T R, R is nil-reflexive. = Suppose that T R, R is a nil-reflexive ring. Let a, b R with arb nilpotent a 0 b 0 for all r R. Then for A =, B = T R, R, ACB = 0 a 0 b arb atb r t is nilpotent for all C = T R, R. By supposition 0 arb 0 r

14 32 HANDAN KOSE, BURCU UNGOR, AND ABDULLAH HARMANCI bra bta r t BCA = is nilpotent for all C = T R, R. It follows 0 bra 0 r that bra is nilpotent for all r R. This completes the proof. We end this paper by studying the Nagata extension of a ring in terms of the nilreflexive property. Let R be a commutative ring, M be an R-module, and α be an endomorphism of R. Let R M be a direct sum of R and M. Define componentwise addition and multiplication given by r 1, m 1 r 2, m 2 = r 1 r 2, αr 1 m 2 + r 2 m 1, where r 1, r 2 R and m 1, m 2 M. This extension is called Nagata extension of R by M and α, and denoted by N[R, M; α] see [11]. Theorem 3.6. Let R be a ring. If R is commutative, then the Nagata extension N[R, R; α] is nil-reflexive. Proof. Assume R is commutative. Let a, n, b, m N[R, R; α]. If a, nx, yb, m is nilpotent for all x, y N[R, R; α], then axb is nilpotent for all x R. By assumption bxa, therefore b, mx, ya, n is nilpotent for all x, y N[R, R; α]. Acknowledgments. The authors would like to thank the referees for their helpful suggestions to improve the presentation of this paper. R [1] F. W. Anderson and K. R. Fuller, Rings and Categories of Modules, Springer-Verlag, New York, [2] R. Antoine, Nilpotent elements and Armendariz rings, J. Algebra, , [3] W. Chen, On nil-semicommutative rings, Thai J. Math., , [4] P. M. Cohn, Reversible rings, Bull. London Math. Soc., , [5] N. K. Kim and Y. Lee, Extensions of reversible rings, J. Pure Appl. Algebra, , [6] T. K. Kwak and Y. Lee, Reflexive property of rings, Comm. Algebra, , [7] T. Y. Lam, A First Course in Noncommutative Rings, Springer-Verlag, New York, [8] J. Lambek, On the representation of modules by sheaves of factor modules, Canad. Math. Bull., , [9] G. Mason, Reflexive ideals, Comm. Algebra, , [10] R. Mohammadi, A. Moussavi and M. Zahiri, On nil-semicommutative rings, Int. Electron. J. Algebra, , [11] M. Nagata, Local Rings, Interscience, New York, [12] M. B. Rege and S. Chhawchharia, Armendariz rings, Proc. Japan Acad. Ser. A Math. Sci., ,

15 NIL-REFLEXIVE RINGS 33 [13] L. Zhao, X. Zhu and Q. Gu, Reflexive rings and their extensions, Math. Slovaca, , Current address: Handan Kose, Department of Mathematics, Ahi Evran University, Kırşehir, TURKEY address: Current address: Burcu Ungor, Department of Mathematics, Ankara University, Ankara, TURKEY address: Current address: Abdullah Harmanci, Department of Mathematics, Hacettepe University, Ankara, TURKEY address: